body > header {
  text-transform: uppercase;
  width: 300px;
  margin: 20px 0;
  color: var(--primary-text-color);
  text-align: center;
  font-size: 20px;
  font-weight: bold;
  position: fixed;
  top: 0;
}

body > header::after {
  content: "";
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  height: 64px;
  z-index: -1;
  background-color: var(--background-color);
}

body > main {
  margin-top: 64px;
  margin-bottom: 90px;
  padding: 0 25px;
  width: 100%;
  max-width: 550px;
  box-sizing: border-box;
  display: grid;
  row-gap: 25px;
}

section {
  background-color: var(--promotion-background);
  display: flex;
  flex-direction: column;
  color: var(--primary-text-color);
  border: 3px solid var(--nav-color);
  border-radius: 15px;
}

section header {
  padding: 15px;
  text-align: center;
  font-size: 20px;
  letter-spacing: 1px;
  text-transform: uppercase;
  font-weight: bold;
  border-bottom: 3px solid var(--nav-color);
}

section ul {
  margin: 15px;
  padding-left: 15px;
  list-style: '✓';
}

section li {
  padding: 10px;
}

section li::marker {
  color: var(--green-color);
  font-weight: bold;
  font-size: 20px;
}

body > footer {
  position: fixed;
  bottom: 0;
  width: 100%;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: center;
  height: 100px;
  background: linear-gradient(
    180deg,
    var(--background-color-alpha) 0%,
    var(--background-color) 25%
  );
}

footer > a {
  width: calc(100% - 50px)!important;
}

@media only screen and (min-width: 900px) {
  body > main {
    max-width: 1100px;
    grid-template-columns: 1fr 1fr 1fr;
    -moz-column-gap: 30px;
         column-gap: 30px;
    min-height: calc(100vh - 180px);
    align-items: center;
  }

  section {
    min-height: 300px;
  }
}

@media only screen and (min-width: 1000px) {
  body > main {
    -moz-column-gap: 50px;
         column-gap: 50px;
  }
}